Add astream_events instrumentation and tests for v1 & v2 in Langchain

[lrafeei]

This PR adds instrumentation and tests for Langchain's astream_events v1 & v2

Separate PR coming for stream_events v3 and astream_events v3

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ ACTION	actionlint	9		0	0	1.18s
✅ MARKDOWN	markdownlint	7	0	0	0	1.2s
✅ PYTHON	ruff	1038	0	0	0	1.12s
✅ PYTHON	ruff-format	1038	0	0	0	0.48s
✅ YAML	prettier	23	0	0	0	1.71s
✅ YAML	v8r	23		0	0	5.37s
✅ YAML	yamllint	23		0	0	0.79s

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 77.27273% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.36%. Comparing base (853d8cd) to head (4292a63).

Files with missing lines	Patch %	Lines
newrelic/hooks/mlmodel_langchain.py	73.68%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1765      +/-   ##
==========================================
- Coverage   82.07%   81.36%   -0.71%     
==========================================
  Files         215      215              
  Lines       26589    26611      +22     
  Branches     4213     4215       +2     
==========================================
- Hits        21823    21653     -170     
- Misses       3315     3519     +204     
+ Partials     1451     1439      -12     

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